Scanning probe microscopes are devices such as AFMs (Atomic Force Microscopes), STMs (Scanning Tunnelling Microscopes) and SNOMs (Scanning Near-Field Optical Microscopes) where operation is based on interaction between a sample surface and a probe in the form of a microfabricated tip. In the AFM, for example, the topography of a sample is sensed using a tip mounted on the end of a microfabricated cantilever to scan the sample surface. Here, the interaction of atomic forces between the nanometer-sharp tip and the sample surface causes pivotal deflection of the cantilever during scanning, and the sample topography is determined by detecting this deflection. This basic principle can be exploited in a variety of operational modes. The tip may be continuously in contact with the sample surface, or the tip may be brought into close proximity with the surface for operation in the so-called “tapping mode”. In some cases, a well-defined force is applied during scanning by application of a voltage between the cantilever and sample. Deflection of the cantilever can be sensed in a variety of ways, for example using piezoelectric or proximity sensors, or using optical detection methods such as laser interferometry.
The AFM technology has also been applied to the field of data storage with a view to providing a new generation of high-density, high data-rate storage devices for mass-memory applications. AFM-based data storage is described in detail in IBM Journal of Research & Development, Volume 44, No. 3, May 2000, pp 323-340, “The ‘Millipede’ —More Than One Thousand Tips for Future AFM Data Storage”, Vettiger et al., and the references cited therein. Here, the cantilever-mounted tip is used to scan the surface of a data storage medium. In a write-scan mode, the tip is used to write data to the surface by creating pits, or bit indentations, in the surface. To write a data bit, a heater on the cantilever is activated to heat the surface at the point of contact with the tip, allowing the tip to penetrate the surface to create a pit. Such a pit represents a bit of value “1”, a bit of value “0” being represented by the absence of a pit at a bit position. In a read-scan mode, the tip is used to read data by detecting the deflection of the cantilever as the tip is moved over the pattern of bit indentations. Here, the cantilever heater, operated at a lower temperature, can conveniently be employed as a proximity sensor since more heat is lost to the storage medium when the tip enters a bit indentation than when no bit indentation is present. Thus, as the tip moves over the bit positions, changes in the temperature-dependent resistance of the heater can be detected to determine the bit pattern. While basic data read/write operations can be implemented in this way using a single cantilever, in practice an integrated array of cantilevers is employed as discussed in the paper referenced above.
Various cantilever designs have been proposed for use in scanning probe microscopy and data storage applications. U.S. Pat. No. 6,079,255, for example, discloses various cantilever designs in which more than one tip is provided on the cantilever. These designs essentially involve two or more mechanically coupled cantilevers: a main, larger cantilever carrying one tip; and one or more smaller cantilevers, each with their own tip, provided within the body of the main cantilever. These designs aim to provide a degree of fine tuning, the main, larger cantilever being used, for example, for low-resolution scans, and the smaller cantilever(s) being used selectively for higher resolution scans. The mechanical coupling of the cantilevers also provides some degree of course position control for the smaller cantilevers in that these are coupled to, and hence follow the movement of, the main, larger cantilever.
In general in scanning probe based devices, some form of alignment procedure is required to ensure proper alignment of the cantilever, in particular to achieve the required orientation of the cantilever and appropriate positioning of the scanning tip relative to the scan surface. In many cases, e.g. where laser interferometry is used for detecting the movement of the cantilever or a force is applied to the cantilever by application of a voltage, the angle of the cantilever should be well-defined with respect to the detecting optical beam or the scan surface. Moreover, depending on the particular application and operating mode, the tip must be brought into gentle contact with the scan surface or into close proximity with the surface. The alignment procedures required to achieve these objectives can be complex and time consuming, and it is often necessary to employ feedback mechanisms to maintain alignment within system parameters. The issue of alignment is particularly problematical where a plurality of cantilevers are operated in parallel as with the data storage array mentioned above. Here, the plane of all cantilever tips should be parallel to the scan surface, but with the integrated array aligned parallel to the surface no further freedom remains for adjusting the orientation of individual cantilevers.